That's the Aero hump. Originally it was for aerodynamic purposes as the name would suggest but it has other functions as well such as hydration storage during the race as it contains a fluid bladder with around 250ml capacity although some riders prefer to run without fluids in order to save weight.

Also, in recent years, leading leather manufacturers Alpinestars and Dainese have taken great strides in airbag technology development for their race suits and now place a small control unit within the hump that monitors G forces and rate of movement to try and predict crashes in order to deploy the airbags in the leathers as soon as possible to help protect the riders. However, this is a recent innovation and not all riders had airbag technology until last year when a new rule was introduced to make Airbag systems mandatory for all manufacturers across all classes.